1. Field of the Invention
The present invention relates generally to radio navigation and guidance and particularly to radio navigation and guidance using broadcast and wireless communication signals without knowing their transmitter locations.
2. Background of the Invention
In modern usage, navigation is confined to the art of determining the translational motion of a vehicle in terms of position and velocity, also referred to as a state vector. When the state vector is calculated on board the vehicle, the process is called navigation. When it is done off board, the process is called position location. Also defined in the book entitled Avionics Navigation Systems (2nd Ed.) by M. Kayton and W. R. Fried, J. Wiley & Sons, Inc., New York, 1997, an additional process is called surveillance when the state vector is measured on board a vehicle and reported to outside the vehicle for off board uses.
Navigation systems are categorized as positioning, which measures the current state vector independent of the path travelled by the vehicle in the past, and dead-reckoning, which derives the state vector from continuous measurements relative to an initial state. An example of dead-reckoning is an inertial navigation system (INS) based on an inertial measurement unit (IMU) using three-axis gyroscopes and accelerometers. Celestial navigation, map-matching, and radio navigation are three popular positioning systems. The present invention is related to a special form of map-matching based radio navigation in a unique way.
Guidance is the art of steering of a vehicle toward a destination. Such handling of a vehicle is also known more specifically as conning for ships, flight control for aircraft, and attitude control for spacecraft. Two forms are identified in the above-mentioned book by M. Kayton and W. R. Fried, namely, (1) toward a designation of known location from the vehicle's present location and (2) toward a designation without explicitly measuring the state vector. One example of the latter guidance law is the proportional navigation guidance in which a seeker captures a target in its field of view (FOV) and a guided vehicle then homes on radio, infrared, and visual emissions of the target until impact. The present invention sets forth a third form of guidance that steers toward a designation represented by location-dependent characteristics of radio signals or of other signals for that matter, without actually defining and knowing the geo-coordinates (state vector) of the destination.
A popular satellite-based radio navigation system is the Global Positioning System (GPS), fully operational since 1994. GPS relies upon a constellation of twenty-four satellites in six different orbit plans around the Earth for position location, navigation, survey, and time transfer. Each satellite carries a set of ultra precise atomic clocks and transmits pseudo-noise (PN) code-modulated signals at several frequencies. By tracking four or more satellites, a user can solve for the variables of longitude, latitude, altitude and time to precisely determine the user's location and clock offset. More details are provided in the book entitled Global Positioning System Theory and Applications (Vols. I and II), edited by B. W. Parkinson and J. J. Spilker Jr., AIAA, 1996.
Despite of its popularity, GPS cannot function well when the line-of-sight (LOS) view between a receiver and GPS satellites is obstructed due to foliage, mountains, buildings, or other structures. To improve GPS receiver sensitivity, one method is the assisted GPS (AGPS). The AGPS approach relies upon a wireless data link to distribute, in real time, such information as time, frequency, navigation data bits, satellite ephemeredes, and approximate position as well as differential corrections to special GPS receivers equipped with a network modem. GPS cannot function well either when GPS signal is heavily jammed or overwhelmed by unintentional interference. GPS signals may be turned off altogether when it orbits over certain region. In such circumstances, no GPS solution is available.
Recently broadcast and wireless communication signals have been considered as signals of opportunity (SOOP) for position location and navigation. Examples of SOOP include digital television (DTV) signals, AM/FM radio signals, mobile phone cellular network signals, and wireless local area network (WLAN or Wi-Fi) signals among others. These signals are designed primarily for indoor reception and in populated areas where GPS often fails to operate properly. There are many inventions disclosed that make use of such signals of opportunity for position location and navigation. This was fueled in part by the U.S. Federal Communications Commission (FCC) mandatory requirement of Emergency 911 (E911) for wireless communications services such as cellular telephone, wideband personal communications services (PCS), and geographic area specialized mobile radio (SMR). Many position location technologies using communication signals have been developed as described in the articles “Standardization of Mobile Phone Positioning for 3G Systems” by Y. Zhao in IEEE Communications Magazine, July 2002 (pp 108-116) and “Network-Based Wireless Location” by A. H. Sayed, A. Tarighat, and N. Khajehnouri in IEEE Signal Processing Magazine, July 2005 (pp 24-41).
However, a prerequisite for navigation and position location using the aforementioned methods, as described in the book entitled Wireless Location in CDMA Cellular Radio Systems, by J. J. Caffery, Jr., Kluwer Academic Publishers, Boston, Mass., 2000, is the accurate knowledge of base station locations for cellular phone network, satellite orbits for GPS, transmitter tower locations for DTV and AM/FM, and access point (AP) locations for Wi-Fi. In general, the locations of radio sources are specified in terms of a set of coordinates relative to a pre-established common reference system. A coordinate system can be an arbitrary grid with its axes referenced to an absolute geodetic coordinate system such as WGS-84. However, there are circumstances in which the location of the signal sources is difficult to determine a priori particularly for indoor environments. Besides, such transmitters as Wi-Fi AP can be easily moved around, thus a database with accurate Wi-Fi AP information is difficult to build and maintain. It is especially true for military and emergency operations for lack of pre-surveyed reference points when using temporary beacons or existing infrastructure in a hostile territory.
Another difficulty for radio navigation in indoor and urban environments is the presence of severe multipath and non line-of-sight (NLOS) signals. In fact, it is more a problem for time of flight (TOF)-based ranging than for communications. A communication system can use, say, a rake receiver to combine signals from different paths to enhance signal to noise ratio (SNR) so as to reduce bit error rate (BER). Sometimes it is even desired to have NLOS signals to reach shadowed areas when the direct path is completely blocked. However, it is the translation of time of flight measurements corrupted by multipath and particularly NLOS signals into range that introduces the most significant errors into positioning, which is difficult to correct afterwards.
Yet, the radio signal propagation channels from fixed radio sources to a stationary receiver remain relatively stable for a given environment. The resulting spatial distribution of signal patterns is akin to standing waveforms produced by reflection, diffraction, refraction, and scattering of radio signals in the environment. Such a quasi time-invariant property of location-dependent radio frequency (RF) signatures, also known as RF fingerprints, has been used for transmitter authentication and position location. RF signatures-based position location consists of two steps. The first step surveys an area of interest by measuring the RF signatures at grid points of known location, thus establishing a database of RF signatures as a function of the grid location. In the second step, the RF signatures measured at an unknown location are compared with the database and when a match is found, the coordinates of the nearest grid point is retrieved (or interpolated) as the user's location. This approach, however, is workable only for those regions for which a survey has been conducted before hand with the location-indexed RF signature database available. More importantly, such RF signatures, though location-dependent, do not carry geometrical information about direction and distance to their sources on which no metrics can be defined and manipulated to yield steering commands.
Migrant birds and sea turtles undertake long-distance travels and repeat their routes year after year. Although animal sensory mechanisms are not fully understood yet, it is evident that the animals do not rely on a common coordinate system to navigate.
A need therefore exists (1) to represent a geo-location with radio signal characteristics measurable at that location, thus taking advantage of radio signal physical and propagation properties that are unique for each location, (2) to navigate based on the radio signal characteristics without converting it into coordinates for that location relative to a coordinate system, and (3) to steer toward a designation using the location-dependent radio signal characteristics without explicitly knowing the radio source locations relative to any geo-coordinate system. This need is met by the present invention as described and claimed below.